1. Field of the Invention
The present invention relates to an image pickup apparatus and a method of controlling the image pickup apparatus.
2. Description of the Related Art
In recent years, a digital camera, a digital video camera and similar devices have been proposed which have a function of detecting a particular subject such as a face of a human figure from an image included in an image signal output from an image sensor, and performing automatic focusing and/or automatic exposure adjustment on the detected subject. Japanese Patent Laid-Open No. 9-149311 discloses an image pickup apparatus having an automatic zoom function in which when a change occurs in distance between the image pickup apparatus and a subject, an automatic zooming operation is performed such that a detected size of an image of the subject is maintained to be substantially equal to a predetermined reference value. This image pickup apparatus starts the automatic zooming operation in response to pressing of a zoom button. The size of an image of a subject detected at the start of the automatic zooming operation is stored as a reference size. During the automatic zooming operation, the image pickup apparatus repeatedly compares the detected size of the image of the subject with the reference size and performs the automatic zooming operation based on a comparison result. That is, after the automatic zooming operation starts, the image pickup apparatus continuously changes the zoom position in a direction to a telephoto end or a wide angle end so that the size of the image of the subject is maintained substantially constant.
In many image pickup apparatuses such as digital cameras and digital video cameras, optical zooming is allowed within a range of several to several tens in zooming ratio. Some image pickup apparatuses have an electronic zoom function to electronically enlarge an image of a subject. However, the electrical enlargement results in degradation in image quality, which determines a practical upper limit on the zoom ratio. The limitation on the zoom ratio of the image pickup apparatus can bring about some problems as described below.
In the image pickup apparatus having the automatic zoom function that is executed by repeatedly comparing the detected size of the image of the subject with the reference size that is set to be equal to the size as of the start of the automatic zooming operation, following problems can occur. When a zoom ratio necessary to achieve the size of the image of the subject to be equal to the reference size is out of a zoom range allowable in the image pickup apparatus, it is impossible to maintain the size of the image of the subject at the reference size. After shooting is further continued in this state, if a change occurs in distance between the image pickup apparatus and the subject, then such a change can cause the automatic zooming operation to become unnatural.
For example, in a case where when the automatic zooming operation is started, the distance between the image pickup apparatus and a subject is small and thus a close-up image of the subject is taken at the start, if the subject moves away from the image pickup apparatus after the automatic zooming operation is started, the image pickup apparatus tries to maintain the size of the image of the subject at a constant value by shifting the position of the zoom lens in a direction toward the telephoto end. However, if the zoom lens reaches the telephoto end, it becomes impossible to further maintain the size at the constant value. As a result, the size of the image of the subject decreases. If a user continues the shooting in this situation in which the size of the image of the subject has become small, the user gets used to the reduced size of the image of the subject. Thereafter, if the subject moves back toward the image pickup apparatus or the user changes the direction of the image pickup apparatus to aim another subject, the image pickup apparatus tries to control the zooming operation to get a close-up image of the subject. As a result, the size of the image of the subject suddenly increases, i.e., an unnatural automatic zooming operation occurs. A similar problem can occur when the position of the zoom lens reaches a wide angle end.
In a case where the image pickup apparatus used is a still camera, and the automatic zoom function is used to achieve a best angle of view in a still image, an unnatural zooming operation such as that described above in terms of the automatic zooming operation may be allowable to a certain degree. However, in a case where a user takes a movie using a video camera or the like, a sequence of images is recorded as the movie in the automatic zoom mode. In this case, it is desirable that the automatic zooming operation is performed such that the size of the subject changes in a natural manner without giving an artificial feeling to a user. That is, such a natural automatic zooming operation is more important in taking a movie by an image pickup apparatus such as a video camera than in taking a still image. In recent years, many digital still cameras have tended to have a movie recording capability, and a natural automatic zooming operation is also important in such a type of image pickup apparatus.
When zooming reaches an upper or lower end of a zooming range and thus it becomes impossible to further control the size of the image of the subject, the image pickup apparatus may display a warning to a user on a display screen such as a liquid crystal display panel. However, displaying such a warning for a long time may annoy the user. To prevent the user from having such a feeling, the image pickup apparatus may stop displaying the warning after the warning has been displayed for a predetermined period. However, this can cause the user to be unaware of the fact that the upper or lower end of the zoom range has been reached and the size of the image of the subject cannot be controlled any more.